This proposal requests funds for the purchase of an additional electron microscope for the School of Biological Sciences Electron Microscope Facility. The Facility provides the space, equipment and technical services for our faculty, postdoctorals, and students. Although there are 78 regular users, there is only one high resolution modern research microscope in the Facility. The major users in this request represent three of the four departments in the School; all of them currently possess NIH funding and all of them have, in the past, and will continue to rely on electron microscopy as a major research technique. The problems studied are varied. They include ultrastructural analysis of chromosomes and chromatin in order to elucidate the relationship between chromatin organization and gene function, as well as the in situ localization of specific DNA sequences along chromosomes and spread chromatin at the electron microscope level; electron microscopic analysis of intermediates in genetic recombination and other allomorphic DNA forms, as well as the analysis of the process of branch migration; mRNA metabolism in Drosophila; the study of synapse formation and the identity and origin of neural explants using ultrastructural and EM histological techniques; and studies on ion and water transport and transepithelial signaling in model invertebrate systems. These studies will provide information which is basic to an understanding of the mechanism of selective gene expression, genetic exchange, and cell division in the normal state; this knowledge is crucial to understanding defects in these processes which are encountered in the diseased state. In addition, techniques pertinent to transplantation of neural tissues will be evaluated. Finally, the studies on epithelial ion and water transport will provide basic information relevant to normal and abnormal kidney function.